1. Field of the Invention
Embodiments of the present invention relates to apparatus and methods processing a substrate with plasma. Particularly, embodiments of the present invention provide a gas distribution plate for supplying processing gas to a plasma processing chamber.
2. Description of the Related Art
During substrate processing, processing gases are generally delivered to a processing chamber through a gas distribution plate. A gas distribution plate usually includes a metal plate having a plurality of through holes or other passages formed therethrough for gas distribution. In a plasma processing chamber, the metal gas distribution plate is often used as an electrode connected to a RF power source or to the RF grounding path for generating capacitively coupled plasma. When a plasma is generated in the processing chamber, ions from the plasma bombard the metal gas distribution substrate, damaging the gas distribution substrate and generating metal particles which may contaminate the substrate during processing.
Traditional attempts to minimize the problems associated with metallic gas distribution plate include coating the metallic gas distribution plate with an inert ceramic coating or bonding a ceramic plate to the metallic gas distribution plate. However, both these approaches has drawbacks.
Ceramic coatings deposited on the metallic gas distribution plate are subject to variation across the gas distribution plate and variation from one gas distribution plate to another. Ceramic coated gas distribution plates may also introduce a higher level of metal contamination as a byproduct of plasma gun deposition techniques which tend to include metal from the gun with the deposited material. The coatings are normally non-volatile and may be sputtered by bombarding ions in the plasma. Additionally, the coated gas distribution plates include gas delivery holes which are exposed to the plasma environment. The plasma naturally forms a sheath, or positive space charge, near any exposed surface. Great design effort is needed to select the aspect ratio of the holes, convoluted line of sight of paths between gas plenum and plasma to prevent localized discharges of gas holes. However, these designs do not work for all pressure regimes, and localized discharge is particularly aggravated when the gas distribution plate is also connected to the RF power source.
Gas distribution plates having a bonded ceramic plate also has a number of drawbacks. First, the electric field generated in the chamber varies radially between the bonded ceramic plate and the metallic gas distribution plate due to differences in the flow of RF current in the space between the metallic plate and the bonded ceramic plate. The presence of the electric field due to the electrical potential difference between ceramic plate and the metallic gas distribution plate increases probability of localized discharge in the space between ceramic and the metallic plates. While great effort can be made to contour the exit hole, but the fact remains these holes have sharp radii greatly increases the electric field concentration in the region between the ceramic and the metallic pieces.
Secondly, the inherent variation of bonding material and bonding process may cause the gas distribution plate to vary laterally in temperature. Temperature control is particularly critical for etch processes where polymerizing gases are used, and any variation in temperature of the gas distribution plate may significantly impact etching performance, for example, loss of the etch profile control of holes and trenches due to poor control of the deposition of polymerizing gases during etching.
Additionally, bonding materials are also vulnerable to erosion due to chemical radical attack from the plasma. This is particularly problematic where metal containing bonding materials are used to increase thermal uniformity because the eroded bonding material then becomes a source of metallic contamination.
Furthermore, the bonded structure can only be used in a limited temperature range. The choice of bonding materials often limits the maximum service temperature of the gas distribution plate. Moreover, the ceramic plate and the metallic plate have coefficients of thermal expansion that are greatly different. When heated, the difference in thermal expansion between the ceramic and metallic plates may cause the bonding material to fail and/or the ceramic plate to crack.
Therefore, there is a need for an improved a gas distribution plate assembly for use in plasma processing.